The assumption that a team full of overachieving stars will perform best also turns out to be unfounded, however. A study by Harvard Business School's Boris Groysberg and others looked at sell-side equity analysts and found that the results of the leading research teams started to wane when the proportion of stars rose above a certain level – broadly, the point at which preening individuals' selfishness and their clamour for more pay impinge on the whole team's results.